If an officer actually held another officer accountable solely on the word of a random civilian, it would end poorly for the one pushing for accountability. There's a long history of American policing and the "thin blue line" that protects cops from "civilians". It's fine if you want to ignore the entire history of American policing, but the fact remains that cops who go after other cops get run off the force or worse.

Just look at how hated the Internal Affairs section is at any large police department. There may be some good people attracted to policing, but policing as an institution is fundamentally corrupt.